An Introduction To Pipe Sizing

September 24, 2014 in Pipe by

For newcomers to the piping industry, pipe sizing can be confusing.

There are various reasons for this, with history probably being the most significant. We still use many aspects of a system that was in place long ago. Since then, technology has evolved and the industry has changed, so what made perfect sense back then might seem a bit awkward today.

However, with a little work, piping sizing isn’t too difficult to understand. Here’s a brief introduction.


The North American system for designating the outside diameter (OD) of a pipe is nominal pipe size, or NPS. Its European equivalent is Nominal Diameter, or DN. The primary advantage of classifying pipes in this way, by their outside diameter, is that it allows pipes of the same size to be fit together even if they have differing wall thicknesses.

The tricky part is this: the NPS value of a pipe doesn’t always equal its OD. Intuitively, you would think that a pipe with a NPS value of ⅛ would have a OD of ⅛ inches. However, it actually has a diameter of .405 inches. The same is true for all NPS values from ⅛ -12; they don’t match up with their OD.

This is because the NPS for these values was originally designed to refer to the inside diameter of a pipe based on wall thicknesses that were standard at the time.

However, for NPS of 14 and above, the NPS value does match up to OD.

Standard Pipe Schedule

In addition to NPS, the other factor to consider when analyzing pipe sizes is pipe schedule. 

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